The UK Border Agency works with Reliance on deportations and was consulted by the company about the welfare of detainees. Photograph: Steve Parsons/PA

The private company hired by the government to deport foreign nationals has decided to place its own guards under surveillance after concluding that some lack respect for ethnic minorities and women and display "loutish" and "aggressive" behaviour.

The damning assessment of the attitudes and conduct of staff working for Reliance is made in an internal company memo, drawn up by senior managers after the company won the Home Office contract to deport foreign prisoners and refused asylum seekers.

The document, one of a number of internal company records leaked to the Guardian, identifies problems "at all levels of the business" and cites poor communication, peer pressure and use of "inappropriate language" by guards empowered to use force to return foreign nationals.

In response, executives at Reliance have decided to recruit a team of covert monitors who will pose as passengers on commercial flights and report back on the performance of guards. They hope the move will quell the growing impression that the deportation system remains in crisis – 18 months after an Angolan man, Jimmy Mubenga, died after being forcibly restrained on a flight from Heathrow.

The Guardian has obtained details of seven further cases of alleged mistreatment of detainees said to have occurred since last May, when Reliance took over the lucrative government removals contract from rival private security firm G4S.

The Home Office said the five allegations it had investigated were found to be "entirely without merit"; in at least one case a detainee is believed to have seriously injured guards during altercations.

Campaigners argue that the complaints process rarely finds in favour of deportees and say the latest complaints indicate a culture of using excessive force remains.

Three G4S guards arrested over Mubenga's death in October 2010 remain on bail. The Crown Prosecution Service is expected to announce whether they will face manslaughter charges at the end of the month.

A parliamentary report last week suggested the removals process was in chaos, as a fifth of foreign prisoners who recently finished their jail terms had still not been deported by last November. The home affairs select committee was highly critical of the UK Border Agency, the Home Office department that works with Reliance, saying it was failing to fulfil its basic tasks and risked damaging public trust.

Reliance consulted independent groups and the Border Agency about the welfare of detainees. The consultation pointed to problems with management of staff and "laddish" behaviour and the memo says there is a variation in the way guards treat detainees. The memo states: "Is there actually a problem with our business? The consensus was: yes. Is this a company where women, ethnic minorities and those of diverse religions feel comfortable? Evidence would suggest: no."

Under a list of reasons why the company may have inherited a damaging culture, the memo identifies complacency and low morale and cites issues to do with supervision, discipline and recruitment. It also concludes there is a lack of respect, shared values and "knowledge of how we should treat people".

In a statement, Reliance said that while it was proud of the overall standards of most staff, it had have begun a programme to rectify some areas of behaviour and attitude. "This will be supported by a programme of on board, covert monitoring of staff behaviour, a process which is due to be introduced in June 2012.

"The company has also reviewed and amended its recruitment processes to encourage more applications from women and those from diverse cultural backgrounds, has restructured the senior management team and created a central unit which has responsibility for overseeing all operational standards."

The company said "an initial period of adjustment" was to be expected when taking on a new contract, adding that the new contract was different from the one held by G4S "in its requirement to balance the welfare of detainees with a more stringent approach to costings and creating more value for money for the taxpayer".

Since taking on the contract, Reliance has embarked on cost-cutting while seeking to change the "culture and behaviour" of guards – many of whom were automatically transferred across from G4S in May. Insiders say the contract has been beset with problems, with guards complaining about shortages of staff, issues with vehicles and poor safety standards.

Eleven guards are understood to have been suspended for refusing to fly on charter flights to Afghanistan, citing a lack of security provisions or risk assessments. On one recent flight to Kabul they say they were left for several hours on the tarmac at the airport, in freezing conditions and fearing for their safety. Reliance said five guards remained suspended for refusing to fly to Afghanistan but insisted its risk assessments had been "stringent".

In a separate incident, a guard was temporarily suspended and given a warning after being overheard making what were perceived as inappropriate comments in an airport lounge in Nairobi, Kenya.

In one of the leaked documents, Reliance blames "austerity cuts" within UKBA for a series of changes in working practices that proved unpopular with staff, some of whom threatened a revolt.

The document warns that its multimillion pound contract with the Home Office is at risk: "Failure to change would make the contract's future unviable," it says.

Reliance consulted widely within the company and with independent groups and the UK Border Agency about the welfare of detainees. The outcome of the consultation pointed to problems with management of staff and "laddish" behaviour and says there is a variation in the way guards treat of detainees. The memo states: "Is there actually a problem with our business? The consensus was: 'yes'."

It adds: "Is this a company where women, ethnic minorities and and those of diverse religions feel comfortable? Evidence would suggest: 'no'."

Under a list reasons why the company may have inherited a damaging culture, the memo identifies "complacency" and low morale and cites issues such issues to do with supervision, discipline and recruitment. It also concludes there is a lack of respect, shared values and "knowledge of how we should treat people".

Reliance recently enlisted the help of Citizens UK, a network of community organisers. Citizens UK decided to work closely with the company, advising on how to implement "cultural change" and improve removals, after it emerged that Mubenga's young children attended a school in group's network of north London primary schools.